1. Field of the Invention
The present invention relates to a ceramic heater and, for example, to a ceramic heater-used in a ceramic glow plug or the like.
2. Description of the Related Art
Conventionally, as a ceramic heater used in a ceramic glow plug or the like, there is known a ceramic heater having a structure in which a heating element for performing resistive heating by current conduction is embedded in an electrically insulating ceramic body containing Si.sub.3 N.sub.4 or AlN as a main component. For example, the heating element is formed from electrically conductive ceramics constituted by silicates, carbonates, borides or nitrides of W, Mo, etc. or from a high-melting-point metal wire of W, W--Re, Mo, etc.
In such a ceramic heater, because the thermal expansion coefficient of Si.sub.3 N.sub.4 or AlN constituting the ceramic body is smaller than that of the conductive ceramic or metal wire constituting a ceramic heating element, stress based on the difference between the thermal expansion coefficients may be concentrated in the ceramic heating element to bring about a problem in the durability of the heater when a cycle of heating and cooling is repeated. Therefore, there has been made an attempt to add a ceramic component such as metal silicate, metal carbonate, metal nitride or metal boride or the like (hereinafter referred to as "adjustment ceramic component") larger in thermal expansion coefficient than Si.sub.3 N.sub.4 or AlN into the ceramic body to reduce the aforementioned thermal expansion coefficient difference to thereby improve the durability.
The aforementioned ceramic heater has a disadvantage in that the following problems arise when the amount of the adjustment ceramic component added to the ceramic body becomes excessive.
(1) Among metal silicate, metal carbonate, metal nitride and metal boride as adjustment ceramic components, there are some components which react with oxygen in the atmosphere so as to be changed into oxides when a cycle of heating and cooling is repeated in a temperature range of from about 400.degree. C. to about 900.degree. C. For example, MoSi.sub.2 is easily changed into oxide when heated to a temperature in a range of from about 400.degree. C. to about 500.degree. C. in the atmosphere. Because of a property in which such oxides are sublimated or evaporated at a considerably lower temperature (for example, in a range of from 700.degree. C. to 800.degree. C.) than the temperature of use of the ceramic heater, pores (vacant holes) may be formed with the sublimation or evaporation of the oxides while the ceramic heater performs heating. There arises a problem that when the content of the adjustment ceramic component becomes excessive, the quantity of the aforementioned pores increases, so that the durability of the heater is lowered.
(2) It is necessary that rare-earth or alkali-earth metal oxide or the like as a sintering assistant agent is added to the ceramic body containing Si.sub.3 N.sub.4 or AlN as a main component in order to accelerate sintering. The sintering property of ceramics containing Si.sub.3 N.sub.4 or AlN as a main component lowers as the amount of the adjustment ceramic component to be added increases. As a result, it become necessary to increase the amount of the sintering assistant agent to be added to compensate for the lowering of the sintering property. However, the heat-resisting property of the ceramic body is lowered as the amount of the sintering assistant agent to be added increases, so that there may be an obstacle to security of durability of the ceramic heater in accordance with the use condition. If the amount of the sintering assistant agent is not increased, the density of the sintered body is insufficient to thereby bring shortage of mechanical strength of the ceramic heater. Furthermore, the amount of the sintering assistant agent to be added has to be adjusted delicately in accordance with the added amount of the adjustment ceramic.
With respect to the conventional ceramic heater, the range of addition of the adjustment ceramic component for avoiding the aforementioned harmful effects has not been sufficiently discussed. Accordingly, there occurred such a problem that the amount of addition of the adjustment ceramic component after became excessive so that the durability of the ceramic heater could not be improved or was lowered, etc.